1. Field of the Invention
The present invention generally relates to a data transmission apparatus and a path control method thereof, using a virtual concatenated (VC) path in an SDH (Synchronous Digital Hierarchy) transmission system, and especially relates to the data transmission apparatus and the path control method thereof, which effectively control the number of virtual concatenated paths to be used in response to an increase and a decrease in an amount of IP data in the data transmission system using the virtual concatenated path method that is effective when the IP data is transmitted by an SDH network.
2. Description of the Related Art
A high order path capacity in the SDH transmission system has been growing with developments of transmission technology, and increased capacity of a transmission signal (payload). Specifically, from VC-3 (Virtual Container, 48,384 kbps), and VC-4 (149,760 kbps) to VC-4-4c (599,040 kbps), VC-4-16c (2,396,160 kbps), VC-4-64c (9,584,640 kbps), VC-4-256c (38,338,560 kbps), etc. have been specified (refer to FIG. 1). Further, a virtual concatenated path transmission system has been developed, wherein a large-volume path signal is divided into a plurality of VC-3/VC-4 signals, transmitted, and restored to the original large-volume path signal at a receiving end (refer to related specifications, such as ITU-T Recommendation G.707 (00/10) and ITU-T Recommendation G.783 (01/1)).
According to this virtual concatenated path transmission system, large-volume data can be divided into 1 through 256 pieces of VC-3/4 data, enabling a transmission of the large-volume data, even if there is no large-capacity transmission-line of such as 10 G bps and 40 G bps available (refer to FIG. 2).
A virtual concatenated path transmission system divides a path container (C-3/4-Xc) into X pieces of VC-3 or VC-4 data, and transmits the pieces via a transmission line. Here, the value of X is set up beforehand by an operator according to data volume to transmit, and VC-3/VC-4path identification numbers used for the virtual connection must also be set up at both a transmitting apparatus and a receiving apparatus.
When the transmitting apparatus transmits a virtual concatenated path, a multi-framing number that is used to absorb a signal transfer-lag, and a sequence number that identifies a VC-3/VC-4 path are provided to the receiving apparatus, using an H4 byte in a path-overhead (POH) of each VC-3/VC-4 (Refer to FIG. 3). The receiving apparatus restores the path container (C-3/4-Xc) by arranging a plurality of received VC-3/VC-4 paths according to the sequence number, and by absorbing the signal transfer-lag of the paths by matching the multi-framing number of the H4 byte.
Here, an example is described. In the case that a signal to be transmitted by an SDH transmission line is a signal of Ethernet (registered trademark), and the like, the Ethernet (registered trademark) signal, unlike a conventional voice signal, tends to have an odd data volume, causing an inefficiency if a path container of SDH is to accommodate the Ethernet (registered trademark) signal. Rather than accommodating the Ethernet (registered trademark) signal in a conventional path container of an SDH transmission line, it is more advantageous in respect of efficiency to use the virtual concatenated (VC) path method that allows setting up a data transmission capacity per VC-3or VC-4. However, even if the virtual concatenated path method is used in an attempt to improve the efficiency, an operator is required to set up an X value on both transmitting side and receiving side, the X value being a number by which data to be transmitted is divided, and determined uniquely by a data volume to be transmitted.
Here, a feature of an Ethernet (registered trademark) signal is in that an amount of transmission traffic increases and decreases in time. In view of the feature, two methods of setting the X value are conceivable,
namely:
(1) setting the X at a value smaller than a maximum signal capacity of the Ethernet (registered trademark) signal, and
(2) setting the X at the maximum signal capacity of the Ethernet (registered trademark) signal.
The method (1) is an advantageous setting method in view of an efficient practical use of an SDH transmission line when usual traffic amount is low. Conversely, if the maximum signal capacity is required, some data will be discarded at an inputting stage to the SDH transmission line. On the other hand, if the setting method (2) is used, although the maximum signal capacity can always be offered, the maximum capacity may not always be used, resulting in useless capacity assigned, and an inefficiency of SDH transmission line usage.
For example, when actual traffic is 80 M bps for an Ethernet (registered trademark) signal capacity of 100 M bps (100 Base), since only a VC-3 (about 48 M bps) and a VC-4 (about 150 M bps are available in the conventional SDH transmission, a VC-4 is provided, resulting in applying the method of (2). The usual traffic is only 80 M bps in this case, resulting in a remaining capacity of about 70 M bps that is assigned uselessly. If the method (1) is applied, two VC-3 paths, also expressed as VC-3-2v and VC-3×2, will be assigned, which provide a suitable capacity for the actual traffic. However, when traffic demand reaches the maximum capacity of 100 M bps, since the capacity of VC-3-2v is 48×2=about 96 M bps, there is an insufficiency of 4 M bps, leaving part of data discarded.